Optical fibers are structures which have an optically transparent core of small diameter and an optical cladding thereover. Protective covers called buffer coatings are applied thereover. In a convenient structure, a silicone coating is applied over the cladding and a teflon layer is applied over the silicone. These buffer coatings serve to protect the fiber cladding from scratching, and they serve to minimize sharp bends, micro-crack formations, and contact with water which causes expansion of micro-cracks when the fiber is stressed. The core and cladding, plus its buffer coatings are considered the optical fiber in the descriptions to follow. Correspondingly, the cladded fiber core, absent its buffer coatings, will be referred to hereinafter as the bare fiber. This fiber is to carry an optical signal therethrough. Fiber optics have existed for thirty years, and their purpose need not be explained.
There is often need to physically anchor the ends and intermediate portions of the fiber in place to support the fiber and to take the stress off of the optical connectors at the ends of the fiber. In other applications, it is necessary to put the optical fibers in tension. Such an application is in remote sensors such as pressure or temperature sensors of the optical interferometer type. In one such sensor, the optical path legs of an interferometer utilize single-mode optical fibers, and one leg of the interferometer is stretched in proportion to the sensed parameter. The use of such optical fibers in an interferometer permits the utilization of a sensing element which can operate in a non-benign environment such as down-hole in a deep oil or gas well where the temperatures are quite high. By using optical fibers in such an interferometer, mechanical ruggedness and long-term reliability are achieved, and a single optical fiber can serve as the two-way data link between the remote sensor and the readout station.
One type of installation in which an attachment of an optical fiber is taught is in E. E. Herman and Bart Likes U.S. Pat. No. 4,859,844, directed to a Comb Filter Pressure/Temperature Sensing System. In that teaching, an optical fiber comprises one leg of a Michelson or Mach Zehnder interferometer in a remote sensing module. In the interferometer, one leg in the remote sensing module is caused to increase in length as a direct function of the sensor parameter, for example, pressure or temperature. This increase in length can be accomplished in the pressure sensor by the use of a Bourdon tube that serves as a transducer converting the sensed pressure to mechanical displacement, thereby causing tension-induced elongation of the optical fiber. The other optical fiber leg of the interferometer serves as a reference. As a function of the resultant differential length between the two legs of the optical interferometer, optical interference occurs at the output of the interferometer where the two legs or optical paths are combined. Various techniques can be used to process this optical signal to provide an unambiguous measurement of the sensed parameter. Remote parameter sensing can thus be accomplished by utilizing some form of transducer in the remote sensing module that translates the sensed parameter into a variation of the differential length between the two legs of the interferometer.
In attaching an optical fiber to a mechanical transducer or fixed attachment point, that fiber must be firmly grasped in a bared region thereof so that it is capable of withstanding considerable tension without slippage or pull-out. Sharp bends and other causes of stress localization in the fiber must be avoided. The attachment device must also be compatible for providing control of fiber tension and fiber rotation during assembly, adjustment, and/or operation. Means must also be provided for mechanically coupling to the attachment device so that tension can be exerted axially on the optical fiber without causing significant bending of the tensioned fiber where it exits from the attachment device.
The requirements of such an optical fiber attachment means in a non-benign sensing application, such as down-hole in a deep oil well, are several-fold. The temperature environment where the remote sensing module must operate in deep oil and gas wells may be as high as 225 degrees C. Long life is important because of the difficulty of replacement. Accuracy must be maintained because calibration may be impossible in the field. The sensing module must also be mechanically rugged. There is need for a proper optical fiber attachment structure to meet such application needs.